In the current technology of the semiconductor integrated circuit, higher integration has been achieved and as a result, the minimum pattern size reaches the region of 100 nm or less. For the formation of fine patterns, exposure technique is regarded as very important, and the exposure technique enables to attain a desired pattern in the following manner. At first, a resist film is applied onto a substrate to be processed (surface to be processed) to which a thin film has been formed, the resist film is selectively exposed with light and then developed so as to form a pattern, a dry etching is performed using the thus obtained pattern as a mask, and finally the resist pattern is removed to obtain the desired pattern.
In order to realize downsizing of the pattern, it is effective to improve and develop both an exposure light source using the shorten wavelength and a resist material of high resolution corresponding to the characteristics of the exposure light source. Currently, ArF excimer laser exposure tools have been on the market. However, these exposure tools themselves are quite expensive and a large scale of cost is expected at the time the exposure tool is updated for the purpose of shortening the wavelength of the exposure tool. Moreover, it is not easy to develop a resist material which corresponds to the shorten wavelength of exposure light, and it is extremely difficult to realize the downsizing of the pattern by only shortening the wavelength of the exposure device.
For these reasons, much attention has been attracted to a new exposure technique, a liquid immersion lithography, in the art. In this method, the space between the projection lens and wafer in the exposure device is filled with liquid having a lager refractive index n than that of air so as to improve and obtain higher resolution than that of the related art.
The resolution of the exposure device is determined by using the following Calculation Formula 1:Resolution R=Coefficient k×Wavelength λ of light source/Numerical aperture NA  Calculation Formula 1
As represented with Calculation Formula 1, the resolution R improves (be smaller), as the wavelength λ of an exposure light source is shorter and the numerical aperture NA is larger. Note that, the numerical aperture of the projection lens is represented as: NA=n×sin α, where n is refractive index of a medium through which the exposure light is transmitted, and α is an angle formed between the exposure light and a light axis of the projection lens. Since the exposure of light is generally performed in atmospheric air, the refractive index n is 1 (i.e., n=1). The liquid immersion exposure method applies the exposure system in which the space between the projection lens and the wafer is filled with a liquid having the refractive index n larger than 1 (i.e., n>1). Accordingly, the refractive index is enlarged from 1 to n (a number larger than 1) in the relative formula of the numerical aperture NA: NA=n×sin α. At the incident angle α of the same exposure light, the resolution R (minimum resolution size) will be reduced in 1/n as NA is enlarged n time(s). In addition, there is also the advantage such that, in the case where the value of NA is set the same, the depth of focus is deepened n times as a can be reduced by enlarging n.
In accordance with the conventional exposure in the air, the numerical aperture NA cannot be adjusted to 1 or larger, as a refractive index of the air between the resist and the lens becomes the factor to limit the numerical aperture NA. However, in accordance with liquid immersion lithography using water, the refractive index relative to the light having a wavelength of 193 nm is increased to 1.44. Therefore, it has been said that the numerical aperture NA can be increased up to about 1.4 on theory. However, as the numerical aperture NA is increased, the angle of the light incident to the resist film is significantly increased. Therefore, the depth of focus (a margin of the focal distance capable of resolution) is reduced. Moreover, to attain higher resolution (the smaller value of the resolution R), liquid immersion lithography of the next generation, which uses a medium having the higher refractive index (n>1.6) than that of water, has been studied. In the case of this liquid immersion lithography of the next generation, it is theoretically possible to increase the numerical aperture NA by about 1.6 times for the exposure in the air, but the current material for an ArF resist has the insufficient refractive index (the refractive index n is about 1.7 to the light having a wavelength of 193 nm). Therefore, the total reflection occurs on the surface of the resist film so that the light does not reach to the inner part of the resist film. As a result, an image cannot be formed, and hence a pattern cannot be formed.
To solve these problem, the studies have been conducted to increase a refractive index of a resist material. However, not so many materials, which can effectively increase a refractive index with maintaining transparency at 193 nm, without impairing acid reactivity desirable for forming a pattern, have not been known in the art. As a familiar example of the material whose refractive index is increased, a resin lens used for glasses and the like has been known. For such material, it is common that the refractive index of the material is increased by introducing heavy metals, aromatic rings, heavy halogen atoms such as bromine and iodine, or sulfur atoms into the material. In the case of the ArF resist material, however, there is a limitation in the method for introducing sulfur atoms because of the transparency at 193 nm, or the problem of contamination.
As prior examples of the resist material whose refractive index is increased, those using a resin containing sulfur, which has a problem in the transparency thereof, disclosed in Idriss Blakey et al., Proc. SPIE, 6519, 651909 (2007), an alicyclic material disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2006-89412, and a curable composition containing aromatic heterocyclic (meth)acrylate disclosed in JP-A No. 2005-133071 have been known. Therefore, it has been desired to develop a material whose refractive index is increased, and which can be easily produced.